1. Field of the Invention
The present invention relates to an improved process for producing 2-methyl-1,4-benzoquinone. More particularly, the present invention relates to a process for industrially advantageously producing 2-methyl-1,4-benzoquinone useful as an intermediate compound for producing Vitamin K by oxidizing m-cresol with molecular oxygen with a high conversion and a high selectivity at a low cost for facilities for the production.
2. Description of the Related Arts
Vitamin K is known as an antihemorrhagic vitamin and includes various types, such as vitamin K.sub.1 (phytonadione), vitamin K.sub.2 (menatetorenone), vitamin K.sub.3 (menadione), and vitamin K.sub.3 sodium hydrogensulfite (menadione sodium hydrogensulfite). Among these vitamins, menadione sodium hydrogensulfite is used in a large amount as an additive to feed.
These types of Vitamin K are prepared by using 2-methyl-1,4-naphthoquinone as the starting material (vitamin K.sub.3 is 2-methyl-1,4-naphthoquinone itself). 2-Methyl-1,4-naphthoquinone has heretofore been industrially produced by oxidation of 2-methylnaphthalene with chromic anhydride. However, this process has a problem in that 6-methyl-1,4-naphthoquinone which is an isomer of 2-methyl-1,4-naphthoquinone is formed at the same time, and efficient separation of 6-methyl-1,4-naphthoquinone is difficult. Therefore, the yield is as low as 20 to 50%.
As the process for producing 2-methyl-1,4-naphthoquinone with a high yield, a process for producing 2-methyl-1,4-naphthoquinone by oxidation of a product of Diels Alder reaction of 2-methyl-1,4-benzoquinone and 1,3-butadiene was recently proposed (Japanese Patent Application Laid-Open No. Heisei 7(1995)-223992). Therefore, 2-methyl-1,4-naphthoquinone can be a very useful intermediate compound for producing vitamin K when 2-methyl-1,4-benzoquinone can be obtained at a low cost.
As the process for producing 2-methyl-1,4-benzoquinone, for example, a process in which o-toluidine is oxidized with manganese dioxide in a dilute sulfuric acid and a process in which m-cresol is oxidized with molecular oxygen are known. When these two process are compared, the process in which m-cresol is oxidized with molecular oxygen is industrially more advantageous because, in the process comprising oxidizing o-toluidine, expensive manganese dioxide must be used and also waste disposal causes problems.
As the process in which m-cresol is oxidized with molecular oxygen, various processes have been proposed. Examples of such processes include (1) a process in which an alkylphenol is brought into reaction with oxygen in a methanol solvent in the presence of a catalyst comprising a copper(II) halide and hydroxide of an alkali metal or alkaline earth metal and an accelerator comprising a halide of an alkali metal or an alkaline earth metal (Japanese Patent Application Laid-Open No. Showa 60(1985)-51144); (2) a process in which a phenol is brought into reaction with oxygen in an alcohol solvent in the presence of copper(II) chloride and an alkali metal chloride (Japanese Patent Publication No. Heisei 6(1994)-57669); (3) a process in which a phenol is brought into reaction with oxygen in a methanol solvent in the presence of copper(I) chloride and an alkali metal chloride (Japanese Patent Publication No. Heisei 7(1995)-42244); and (4) a process in which a phenol is brought into reaction with oxygen in a methanol solvent in the presence of copper(II) chloride and an alkali metal chloride (Japanese Patent Publication No. Heisei 7(1995)-76192).
However, process (1) is not always satisfactory because the conversion is small in the oxidation of m-cresol although the reaction condition is relatively mild. Processes (2) to (4) have drawbacks in that these processes require a high pressure of oxygen (for example, the preferable pressure of oxygen in process (2) is 20 to 150 kg/cm.sup.2, the required pressure in process (3) is 30 kg/cm.sup.2 or higher, and the preferable pressure in process (4) is 30 kg/cm.sup.2 or higher), which inevitably causes a higher cost of facilities for the production, and that no example can be found about using m-cresol as the phenol.